1. Field of the Invention
The present disclosure is related to hydraulic pump/motors operating opposite one another, and in particular to independent control of opposed pump/motors for improved operational efficiency.
2. Description of the Related Art
In recent years, significant interest has been generated in hybrid vehicle technology as a way to improve fuel economy and reduce the environmental impact of the large number of vehicles in operation. The term hybrid is used in reference to vehicles employing two or more power sources to provide motive energy to the vehicle. For example, hybrid electric vehicles are currently available that employ an internal combustion engine to provide power to a generator, which then generates electricity to be stored in a battery of storage cells. This stored power is then used, as necessary, to drive an electric motor coupled to the drive train of the vehicle.
There is also interest in the development of hybrid hydraulic vehicles, due to the potential for greater fuel economy, and a lower environmental impact than hybrid electric vehicles. According to one configuration, a hybrid hydraulic vehicle employs an internal combustion engine (ICE) to drive a hydraulic pump, which pressurizes hydraulic fluid. The pressurized fluid is stored in an accumulator and later used to drive a hydraulic motor coupled to the drive wheels of the vehicle.
Hydraulic hybrid vehicles and their operation is described in a number of references, including U.S. Pat. No. 5,495,912 and U.S. patent application Ser. Nos. 10/386,029; 10/672,732; and 10/769,459. Each of the above listed references is incorporated herein by reference in its entirety.
There is a class of hydraulic machines commonly employed in hybrid operation that includes a rotating barrel having a plurality of cylinders, and pistons reciprocating within the cylinders. The barrel is configured to rotate over a valve plate having inlet and outlet ports. The barrel rotates over the valve plate, and fluid passes into, and out of, the cylinders of the barrel. In a hydraulic pump, fluid is drawn into each cylinder from a low pressure inlet port and forced out of the cylinder to a high-pressure outlet port. In a hydraulic motor, fluid from a high-pressure inlet enters each cylinder in turn and vents to a low pressure outlet. Some machines, commonly referred to as pump/motors, are configured to operate as pumps or motors, according to how fluid is applied to the machine.
The operation of a typical bent-axis pump/motor will be described with reference to its operation as a motor. Operation of such devices in “pump” mode will not be described inasmuch as such operation will be clear to one having ordinary skill in the art, in view of the following description. For brevity, pump/motor machines may be referred to hereafter simply as motors.
In the accompanying figures, some of the features are shown as being members of a plurality of substantially identical features, in which case, each of the plurality is given the same reference number. In cases where a letter is also used, this is for ease of reference, for the purpose of indicating particular ones of the plurality in the descriptive text. Use of the reference number without specifying an accompanying letter may be understood to indicate the corresponding features generically.
The term “axial force” is used herein to refer to force vectors that lie substantially parallel to a defined axis, while the term “radial force” is used to refer to force vectors that lie in a plane that is substantially perpendicular to a defined axis. Neither term is limited to vectors that intersect the axis. In particular, the radial forces referred to herein generally lie in vectors some distance from the defined axis such that a device that is configured to rotate about the axis, and upon which the radial forces act, will tend to rotate in reaction to the forces.
FIGS. 1A-1C show sectional views of a portion of a bent-axis pump/motor 100 according to known art. The motor 100 includes a valve plate 102 and a cylindrical barrel 104, having a plurality of cylinders 106 within which pistons 108 travel reciprocally. The pistons 108 each have a sliding seal engagement with walls of the respective cylinder 106, at first ends of the pistons. Each of the pistons 108 engages a respective socket formed in a drive plate 110 at a second end thereof. The drive plate 110 is coupled to an output shaft 120 that is rotationally driven by the motor 100. The drive plate 110 bears against a thrust bearing 118 configured to permit free rotation of the drive plate 110 and shaft 120, while holding the drive plate in position against radial and axial forces acting thereon. Typically, bent-axis pump/motors are provided with an odd number of cylinders and pistons, usually seven or nine. In FIGS. 1A-1C cylinder 106A and piston 108A are shown positioned at the top of the barrel 104 while cylinder 106B and piston 108B are shown at the bottom of the barrel 104. In the case of an actual machine employing an odd number of cylinders, a cross-section through a center of the barrel would not intersect two cylinders at the same time, but that condition is shown here for the purpose of illustrating the relative volumes of fluid constrained by the pistons 108 at the top and bottom of rotation.
The cylinder barrel 104 is configured to rotate around a first axis A with a face 114 of the cylinder barrel 104 slideably coupled to a face of the valve plate 102, which does not rotate. The drive plate 110 rotates around an axis B, and is coupled to the rotating cylinder barrel 104 by a constant velocity joint 116 (only portions of which are shown in FIGS. 1A-1C). Accordingly, the cylinder barrel 104 and the drive plate 110 rotate at a common rate.
As the cylinder barrel 104 rotates, each of the cylinders 106 follows a circular path. The uppermost point of that path is referred to as top-dead-center, indicated in FIGS. 1A-1C as TDC, while the lowermost point in the rotation is referred to as bottom-dead-center, indicated in FIGS. 1A-1C as BDC.
The valve plate 102, barrel 104, and pistons 108, which define axis A, are configured to rotate with respect to the drive plate 110, which defines axis B, for the purpose of varying the displacement volume of the pump/motor 100, as explained below. The degree of rotation of axis A away from a coaxial relationship with axis B is typically referred to as the stroke-angle of the device.
FIG. 1A shows the motor 100 at a maximum stroke angle, which provides a maximum displacement of the motor for a high degree of energy transfer. FIG. 1B shows the motor 100 positioned at a moderate stroke angle, and FIG. 1C shows the motor 100 at a stroke angle of zero, wherein the axes A and B are coaxial, and wherein energy transfer is virtually zero.
The term displacement is used to refer to the volume in the cylinders 106 that is swept by the pistons 108 during a single rotation of the barrel 104, and may be used with a numerical value and a unit indicating a volumetric measure, such as cm3, etc., when referring to a particular machine. In the present case, the devices pictured in the accompanying figures are provided for the purpose of illustrating principles that are important to an understanding of the invention, and are not intended to depict specific devices. Accordingly, volumetric values of displacement will not be provided.
In each of the FIGS. 1A-1C, the piston 108A positioned in cylinder 106A at TDC, lies at the inner limit (IL) to which it will travel over the course of a rotation of the barrel 104, given the stroke angle shown. The position of the face of the piston 108A is indicated at line IL. By the same token, the piston 108B positioned in cylinder 106B at BDC, lies at the outer limit (OL) to which it will travel over the course of a rotation of the barrel 104. The position of the face of the piston 108B is indicated at line OL. In any given cylinder 106, the volume that lies between the lines OL and IL represents the displacement of that cylinder 106. Thus, the displacement of the pump/motor 100 is the sum of the displacements of all of the cylinders 106 of the device at that stroke angle.
When the stroke angle is at a 100% displacement, as shown in FIG. 1A, the lines OL and IL lie a maximum distance apart. This is the maximum displacement that can be achieved by the pump/motor 100, and provides the highest degree of energy transfer from the high-pressure fluid to the rotation of the drive plate 110, in the form of torque. FIG. 1B shows a moderate stroke angle of approximately 50% displacement. It may be seen that the lines OL and IL lie closer together than in FIG. 1A. At this lower angle, a lower degree of force transfer is achieved. When the pump/motor is at a minimum stroke angle, as shown in FIG. 1C, the lines OL and IL define the same point since, as the barrel 104 rotates while at this stroke angle, the pistons 108 do not move axially within the respective cylinders 106, and so do not sweep any volume. At the stroke angle shown in FIG. 1C, the drive motor 100 is at minimum, or 0%, displacement, and receives no motive force from the high-pressure fluid, but is free to coast under its own inertia or by external forces. When a motor is destroked to 0%, the high-pressure fluid supply may be closed so that the pistons are not subject to the high pressure. This eliminates axial loading by the pressurized pistons, as will be described in more detail hereafter.
When the pump/motor 100 is operating in a motor mode, high-pressure fluid is valved into each cylinder 106 as it passes TDC. The high-pressure fluid applies a driving force on the face of the piston 108 that acts on the piston 108 axially with respect to axis A. This force is transferred by the piston 108 to the drive plate 110.
Referring to FIG. 1A, it may be seen that the driving force on the pistons 108 is axial, relative to axis A, but will include both axial and radial force components, relative to axis B. The distribution of the driving force between the axial and radial components will depend on the stroke angle of the pump/motor, and can be calculated in accordance with well known and long established mechanical principles. The axial component will tend to drive the drive plate away from the barrel along axis B, which is prevented by the thrust bearing 118. The radial component will tend to drive the socket of the drive plate 110, into which the second end of the piston 108 is seated, to move downward, causing the drive plate 110 to rotate so that the socket moves further away from the barrel, with the barrel 104 rotating in unison.
It will be recognized that the lower the stroke angle, the more of the driving force will be distributed to the drive plate as an axial force, until, at a zero stroke angle such as that shown in FIG. 1C, all of the drive force is distributed to the drive plate as an axial force. It will also be recognized that, while the radial force component serves to drive the rotation of the drive plate to provide torque to the motor, the axial force against the drive plate is not converted to a useful form of energy, and tends to exert high loads to the drive plate 110, the bearing 118, and the valve plate 102, and create frictional resistance to rotation of the motor 100. As a consequence, the efficiency of the motor 100 in converting fluid pressure to a useful form of energy decreases as the stroke angle decreases, and increases as the stroke angle increases.
A more detailed discussion regarding the operation and structure of hydraulic pump/motors may be found in U.S. patent application Ser. No. 10/379,992, entitled HIGH-EFFICIENCY, LARGE ANGLE, VARIABLE DISPLACEMENT HYDRAULIC PUMP/MOTOR; and Ser. No. 10/795,797, entitled EFFICIENT PUMP/MOTOR WITH REDUCED ENERGY LOSS, which applications are incorporated herein by reference, in their entirety.